1. Field of the Invention
The present invention relates to an I/O pin used for electrically connecting printed boards characterized in that one end of the I/O pin is perpendicularly secured onto one printed board, and the other end of the I/O pin is soldered to a predetermined position on the other printed board when the one printed board is mounted on the other printed board.
2. Description of the Related Art
A multichip module, which will be referred to as MCM hereinafter in this specification, composed of a small printed board on which semiconductor elements (bare chips) are mounted is mounted onto a large printed board (mother board) via a large number of I/O pins, the diameter of which is approximately 0.2 mm and the length of which is approximately 3 mm.
FIG. 11 is a view showing a primary portion of an example of the structure in which the small printed board is mounted onto the large printed board.
Referring to the drawing, on the surface of mother board MB, a plurality of disk-shaped pads Pd are formed by means of etching, wherein only one disk-shaped pad Pd is shown in the example shown in the drawing. On the pad Pd, a layer of solder S and a layer of flux (not shown in the drawing) are laminated in this order.
On the other hand, an MCM includes: a ceramic substrate CB on which six layers (three layers in the example shown in the drawing) of polyimide Pi are usually formed, between which wiring layers are interposed; a plurality of bare chips (semiconductor elements) BT (one bare chip is shown in the example shown in the drawing) mounted on the ceramic substrate; and I/O pins 51a, the number of which may be several hundred to several thousand (one I/O pin is shown in the example shown in the drawing), by which an MCM and a mother board are connected to each other.
When an MCM is mounted on a mother board MB, I/O pin 51a collides with pad Pd, which corresponds to I/O pin 51a, and is temporarily attached to mother board MB by a layer of flux. After that, I/O pin 51a is soldered (secured) to the corresponding pad Pd by solder S which is melted by a reflow treatment.
When an MCM is mounted on a mother board MB, the following problems may be encountered. Solder S creeps upward along I/O pin 51a, that is, solder S flows from a fore end to a base of the I/O pin and extends to the polyimide layers Pi on MCM. At this time, solder S intrudes between the polyimide layers Pi. Therefore, the MCM suffers damage that can not be repaired.
When MCM is heated so that it can be detached from mother board MB, there is caused a problem that solder S flows, that is, solder S creeps upward, and this phenomenon was described before.
Further, when bare chip BT is detached after MCM has been removed, solder S depositing at the fore end of I/O pin flows to the base side of I/O pin, that is, solder S creeps upward, and the same problem may be encountered.
In this connection, techniques disclosed in the patent publications relating to the present application will be briefly introduced as follows.
Japanese Unexamined Patent Publication No. 63-74657 discloses a soldering structure by which IC for driving and a substrate are soldered to each other. In this structure, there is provided a layer (dam) for preventing the diffusion of solder in the periphery of the pad portion of the substrate.
Japanese Unexamined Patent Publication No. 62-165960 discloses a structure of soldering in which a surface mounting J-bend type IC such as PLCC (Plastic Leaded Chip Carrier) and a substrate are soldered to each other. At the base portions of a large number of leads, which are bent into a J-shape, there are formed layers (dams) made of resin for preventing solder from creeping upward.
Japanese Unexamined Patent Publication No. 52-51569 discloses a method of manufacturing a substrate of polyimide. On a copper layer pattern formed on the substrate, a layer (dam) for preventing the diffusion of solder is formed by the photo-resist method.
Japanese Unexamined Patent Publication No. 2-187045 discloses a structure of the pad arranged on the substrate on which IC chips are mounted. In this structure, an oxide film (dam) of low solder wettability is formed in the periphery of the pad by heat treatment.
Japanese Unexamined Patent Publication No. 2-47890 discloses a structure of a circular pad on the substrate with which the pins of electronic parts are made to collide. A small circular layer (dam) made of material, to which no solder adheres, is formed at the center of the pad so that the pin can be aligned.
Japanese Unexamined Patent Publication No. 58-35996 discloses a structure of a substrate having a through-hole into which a pin insertion type IC is inserted. On a surface of the substrate opposite to the side on which IC is mounted, a sublimation type solder-resist (polyurethane) is coated for the prevention of diffusion of solder.